When light hits chlorophyll a few things can happen
When pigments absorb photons an election gets excited
Light energy to chemical energy
Light reactions: make ATP + NADPH for Calvin cycle
Calvin cycle (in stroma)
Calvin cycle is a 3-step process
- 1. Fixation: CO2 reacts with 5C molecule (RuBP)
- catalyzed by Rubisco enzyme
- new 6C split into 2 acids (3C each)
- 2. ATP and NADPH turn 3C acids into sugars
- reduction of acids
- 3. Some 3C sugars recycled to re-make RuBP (5C)
- one 3C sugar leaves to make glucose
- recycling needs ATP
Making sugars requires several cycles
Rubisco - the enzyme that dominates carbon fixation
- Most abundant enzyme in the world
- Most CO2 converted into biomass is fixed by Rubisco
- Large molecule : 16 polypeptides : 8 active sites
Rubisco: the clunky and slow carbon fixer
- Only 3-10 reactions per second
- limits rate of photosynthesis
- 20% error rate
- gets worse at high temperatures
- Also reacts with O2 (cheater!)
- O2 reaction called Photorespiration
- uses ATP and NADPH to make CO2
C3 photosynthesis how did we get here…
- Evolution doesn’t always create the best solution
- Photorespiration is completely wasteful
- 2x energy to produce the same amount of sugar than if Rubisco only reacted with CO2
- Rubisco has evolved high affinity for CO2 (80x > O2)
- compare to the composition of the atmosphere…
Rubisco and plant evolution: How did we get here?
- Why is there so much Rubsico?
- Why is it so big and slow?
- Was Rubisco always a cheater?
- Why did the atmosphere change?
Why has Rubisco not been replaced?
- Evolution of enzymes difficult
- favors stability
- selection against modifying mutations
- RubisCO → RubisC???
- requires a decrease in fitness
- plants are stuck with Rubisco
- Natural selection increased affinity for CO2
- made Rubisco slow
Evolution tinkers with other parts of photosynthesis
- Changed existing parts and pieces
- work around the photorespiration issue
- In response to environmental change
- lower [CO2] & higher [O2]
- high temperatures
- water limitations (stomata)
- Evolution of new photosynthesis pathways
- Allowed plants to exploit drier habitats
Evolution of C4 photosynthesis
- Calvin cycle moved to Bundle Sheath Cells surrounding leaf veins
- chloroplasts now present
- BSC cells are less permeable to gases
- what does this mean for photorespiration?
C4 plants concentrate CO2 around Rubisco spatially
- C4 mechanism separates Calvin cycle from high O2
- New fixation pathway in mesophyll cells
- New enzyme, phosphoenolpyruvate carboxlylase (PEPC)
- fixes CO2 to PEP (no affinity for O2)
- new 4C malate pumped to bundle sheath cells
- malate broken down to release CO2
- 3C molecule transported back to mesophyll
- Hyper efficient with use of CO2
- How does this impact stomata behavior?
Why is C4 photosynthesis not dominant?
- Regeneration of PEP expensive (ATP)
- in addition to ATP needed for Calvin cycle
- Where is needed ATP generated?
- Where should C4 plants exist?
In hot conditions, the benefits of reduced photorespiration likely exceed the ATP cost of moving CO2 from the mesophyll cell to the bundle-sheath cell
Evolution of CAM photosynthesis (C4 pathway w/out BSC)
CAM concentrates CO2 around Rubisco temporally
- Stomata open at night only
- 4C compounds produced in large quantities
- stored when Rubisco is inactive
- C4 vs CAM
- substitute BSC for “night shift”
Yes we CAM!!
- CAM evolved independently several times
- ~16,000 species
- Exist in arid environments or habitats
- cacti
- epiphytes
- Are CAM plants fast or slow growing?
C3 vs C4 vs CAM: Know advantages/disadvantages of each
Why does this matter to you…
- C4 plants are economically important
- corn, sugarcane, sorghum & switchgrass
- So are C3 crops
- beans, rice, wheat, potatoes (temperate crops)
- Global water use is set to triple by 2050
- 70% to the agriculture sector
- 50% transpired through stomatal pores
